Recently, mobile radio apparatuses such as ETC (Electronic Toll Collection) systems have come into wide use. As antenna apparatus to be used in such radio apparatuses, there are proposed ring antenna apparatuses not employing an expensive dielectric substrate.
FIG. 5 is a perspective view of an example of such an antenna apparatus and FIG. 6 shows a circuit diagram of the same.
The conventional antenna apparatus is constructed of wiring substrate 40, ring electrode 43, and matching circuit 60, with, in addition, coaxial line 51 and signal circuit unit 50 connected to the same. Wiring substrate 40 has grounded electrode 41 made of copper foil formed on its top face and wiring pattern 45 in a predetermined shape formed on its rear side. Ring electrode 43 in a ring shape is supported by a plurality of supporting members 42 so as to oppose grounded electrode 41 at a predetermined distance from the same. Further, feeding terminal 44, which is provided to pass through wiring substrate 40 having grounded electrode 41 formed thereon, and ring electrode 43 are electrically connected at the position of connecting point 431 of ring electrode 43.
Matching circuit 60 has a function (hereinafter called “impedance matching”) to allow a high-frequency signal to be transferred without loss from signal source 50 to ring electrode 43 via feeding terminal 44. Matching circuit 60 is made up of pluralities of coils 47 and capacitors 48 as electronic components for general purpose. Matching circuit 60 is connected with wiring pattern 45 on wiring substrate 40. Further, matching circuit 60 is connected to coaxial line 51 and coaxial line 51, in turn, is connected to signal circuit unit 50.
As shown in FIG. 6, matching circuit 60 is configured such that coil 47 is connected, in series, between signal source 50 and ring electrode 43, while capacitor 48 is connected, in parallel, at its one end with wiring pattern 45, disposed between coil 47 and connection point 451 with feeding terminal 44, and at its other end with grounded electrode 41. Incidentally, one end of wiring pattern 45 in connection with coil 47 is connected with coaxial line 51 at predetermined connection point 511 of wiring pattern 45. Further, the other end of feeding terminal 44 is connected to connection point 431 of ring electrode 43 as earlier mentioned.
When a radio wave is transmitted from the antenna apparatus in the above mentioned configuration, a high-frequency signal from signal circuit unit 50 is transferred by means of center conductor 52 and outer conductor 53 of coaxial line 51 and input to matching circuit 60 of the antenna apparatus. In matching circuit 60, impedance matching is performed, and the high-frequency signal passed through matching circuit 60 is transferred to ring electrode 43 and transmitted as a radio wave. When a radio wave is received, operations reverse to the above are performed so that a signal is received by signal circuit unit 50.
FIG. 7 is a Smith chart showing impedance matching characteristics. Description will be given below with use of the chart. Suppose now, for example, that the impedance of the antenna apparatus without matching circuit 60 inserted therein (hereinafter referred to as “initial impedance”) is positioned at point 561 to the right of center point 55 (=50Ω) on the horizontal center line of the Smith chart. If, then, matching circuit 60 is inserted, the following change in the situation occurs. Namely, by virtue of matching circuit 60 formed of coil 47 and capacitor 48, the impedance makes rotational movement in a clockwise direction along the circumference of what is called constant-conductance circle in the Smith chart to change its position to point 562. Then, from the position of point 562, it further makes rotational movement in the clockwise direction along the circumference of what is called constant-resistance circle to change its position to center point 55 (=50Ω) and, thus, impedance matching is achieved.
In such conventional antenna apparatuses, matching circuit 60 for impedance matching is made up of capacitors and coils as the electronic components for general-purpose. However, such capacitors and coils have predetermined stepped values. As a result, in order to achieve proper matching with use of such electronic components having predetermined stepped values, it is necessitated to use pluralities of capacitors and coils. Hence, there has been a problem that the number of components forming matching circuit 60 increases.
In view of the problem, there is disclosed a loop antenna for circularly polarized wave In Japanese Patent No. H04-67363. This antenna has a grounding plate and a loop conductor, provided with precession elements for emitting circularly polarized wave, arranged to oppose each other with a dielectric member sandwiched therebetween. Further, there is provided an L-shaped element extending a predetermined distance from a predetermined position of the loop conductor toward the interior of the loop, while the center conductor of a power supplying coaxial line is connected to the L-shaped element and the outer conductor of the same is connected to the grounding plate. Although this antenna apparatus is not using a capacitor or coil as an electronic component for general purpose, it is using a dielectric substrate. Hence, it is considered relatively difficult to manufacture this apparatus at low cost.
Further, in Japanese Patent Unexamined Publication No. H04-88702 is disclosed a configuration, in a matching circuit for an antenna to be interposed between a narrow band antenna and a power supplying wire to thereby match the antenna with the power supplying wire over a wide band, having a first standard impedance wire with a predetermined length disposed on the side of the antenna, having a second low impedance wire with a predetermined length and a third high impedance wire with a predetermined length serially connected, in series, to the standard impedance wire, and, further, having a second low impedance wire with a predetermined length connected, in series, to the high impedance wire on its side toward the power supplying wire. This configuration is also not using a capacitor or coil as an electronic component for general purpose, and provides an antenna apparatus of simple structure and small size. However, it seems to be a problem with this type that it, due to its configuration, becomes high-profiled as an antenna apparatus.
The present invention has been made to solve such problems with conventional types and, accordingly, it is an object of this invention to provide an antenna apparatus not requiring a capacitor or coil as an electronic component for general purpose, allowing precise adjustment of the impedance matching to be made easily, and being inexpensive.